1. Field of the Invention
The present invention relates to drive motors for providing power for operating injection molding machines, and more particularly to electric motor drive systems utilizing two electric motors mechanically coupled together.
2. Description of the Related Art
For a number of years injection molding machines for molding plastic parts were hydraulically operated. The machines included linear hydraulic actuators for providing linear motion and rotary hydraulic motors for providing rotational motion. As the need for greater precision of positioning of the respective elements of such machines during an operating cycle increased it became apparent that hydraulically-powered elements were limited in positioning precision because of the compressibility of hydraulic fluids and the volumetric changes of the fluids with changes in fluid temperature. Accordingly, several years ago electric motors began to be utilized in mechanical drive systems for such machines, principally because of the closer position control that can be obtained through the use of shaft angle encoders and other electrically-operated position-sensing devices. In particular, servomotors provide the desired degree of control precision by virtue of the ability to closely control shaft angles and current supply, enabling close control over motor shaft torque, shaft position, and shaft angular velocity, either by open-loop or closed-loop control techniques.
In addition to the ability to more precisely control an electrically-powered device as compared with an hydraulically-powered device, hydraulic devices are often more noisy than electrical devices. Further, considerable energy is wasted in hydraulic machines by reason of the heating of the hydraulic fluid that occurs as the hydraulic fluid is pressurized during operation of the device, and the consequent need to cool the fluid by means of oil or water coolers or other forms of heat sinks. And in this era of greater attention to environmental aspects, the mineral oils in hydraulic fluids are sometimes environmentally undesirable, requiring special containers and special disposal techniques.
One disadvantage of electric motors is that when higher power output is required the size, weight, and cost of electric motors is greater that that for hydraulic motors. Because the degree of control precision is so much greater for electric motors as compared with mechanical and hydraulic systems, however, electric motors are becoming more and more ubiquitous, particularly in smaller capacity injection molding machines, those having clamp tonnages of up to about 300 tons. When larger outputs are required the limitations of power output and the increase in the diameters for larger electric motors has limited the sizes of injection molding machines with which electric drive systems can be employed.
AC servomotors are often utilized in powering injection molding machines. However, commercially available AC servomotors are limited with respect to maximum output torque. Thus, the capacity of an injection molding machine with which such AC servomotors can serve as the power source is limited by the output torque of commercially available motors. And although two such motors can be coupled by means of belts or gears to provide combined motor torque output, such systems occupy additional space and become more complex. Additionally, such systems also add mechanical elements that reduce the precision of control of the output from the combined motors because of belt stretch, mechanical wear, and the like factors that are inherent in mechanical drive elements.
It is an object of the present invention to provide a drive system whereby electric motors can be employed to power machinery where greater maximum torque outputs are required than can be obtained using a single commercially-available motor.
It is another object of the present invention to provide a drive system whereby the overall size of an electrically-powered injection molding machine requiring a high output torque electric motor is maintained within desired space limitations.